Transistor amplifiers have a peak efficiency for a particular input power that is a function of geometry (i.e. circuit components and layout), load and supply voltage. In conventional radio frequency (RF) power amplification these characteristics are fixed based on the peak input level expected. For amplifiers presented with an input signal having a wide dynamic range, the input signal infrequently achieves peak levels and frequently operates below peak levels. As such, the amplifier may exhibit low overall efficiency.
A solution to the problem of low amplifier efficiency is to vary one or more of the above-stated characteristics (geometry, load, supply voltage) in response to the input signal. Techniques to vary one or more of these characteristics are known in the art.
Techniques that vary the device geometry and load tend to be very dependent on the particular power amplifier topology used, and generally present challenging RF problems. Repeatability of such designs in production is generally a problem.
Various techniques are known in the art for enhancing amplifier efficiency based on the supply voltage. Of supply voltage based efficiency enhancement schemes, there are two broad classifications of solution. These solutions are:                (i) envelope elimination and restoration, and        (ii) envelope tracking.        
Envelope elimination and restoration requires the amplifier to be driven saturated, and all the envelope information to be applied through the amplifier supply. This technique tends to be generally too demanding upon the supply modulator when using high modulation bandwidths, and thus has limited usefulness in practical applications.
With envelope tracking, the amplifier is driven in a substantially linear fashion. Envelope tracking requires an efficient power supply capable of delivering high modulation power bandwidths. In known techniques, a switched mode pulse width modulator (commonly referred to as class S) is used to realise an efficient variable supply to the power amplifier. However, in order to operate at full bandwidth, the supply must switch at many times the bandwidth of the modulation, and this excessively high switching speed results in poor modulator efficiency.
In another prior art envelope tracking technique, a plurality of highly efficient intermediate power supplies are provided, and the power supplies are switched as required by the envelope level. This switching creates transient disturbances that degrade the spectrum with high order intermodulation products, and makes linearization difficult by introducing supply dependent non-linearities alongside input dependent non-linearities.
In a further modification to this technique, the switching of the power supplies is combined with a linear amplifier to provide a smooth transition between switch levels and remove the supply dependent linearization requirement. The aim of this form of envelope tracking is to provide a unique value of supply voltage for every envelope level. However, there is a problem in achieving this without impact upon tracking speed capability.
It is an aim of the present invention to provide an improved supply voltage based efficiency enhancement scheme, which preferably addresses one or more of the above-stated problems.